The invention relates to a method for manufacturing a projection weld connection for electrically conductive plate material, such as body parts of automobiles, and parts of plate material for household appliances, such as washing machines, refrigerators, and such.
A method and device of this type is known from U.S. Pat. No. 4,417,122.
This patent concerns a method for manufacturing a projection weld connection for plate material in which a projection is performed in one of the plate parts to be welded, during which the plate parts are compressed in a pressure unit and a pulse of current of a short duration is applied.
This patent concerns the so-called projection welding process, whereby one of the two parts to be welded carries a protruding projection which is pressed with a movable electrode against a fixed welding head, during which the projection welds together against the other plate.
The electrical energy is controlled by means of a circuit during a very short time through the connection to be manufactured.
A great disadvantage with this method is that the electrode leaves a visible imprint in the outer plate surface. These imprints need to be further finished, which on the one hand can take place by grinding, which comprises an extra and expensive manual operation.
On the other hand, this grinding is undesirable for plate surfaces that have been finished with an anti-corrosion surface treatment such as galvanizing or electroplating.
The object of the invention is to provide a method and device for manufacturing a projection weld connection, which does not leave any traces in the visible part, and leaves a strong structural connection capable of making a sturdy connection of the parts without any additional means, such as adhesive, being necessary.
This object is achieved according to the invention in that an oblong projection is formed. whereas the plate parts are pressed together from one side at the location of the projection with a pressure unit applying the one-sided force of between 300 and 1800 Newtons, and the weld is made with a pulse of current of about 15,000-50,000 Amperes during 1 to 10 milliseconds which current is supplied to the overlying plate part from the side of the pressure unit and is removed from the underlying plate part.
For this reason use is made of projections pressed in one of the plate parts, said projections being melted during welding in such a way, that on the opposite side no traces are formed.
Preferably the projections are pressed in the underlying plate at the location of the connection to be manufactured, by which the force of the pressure unit can be kept limited.